Writing a mathematical work in english this question is less about mathematics, but I don't know else where to ask.
I am native to german and have to write my current thesis in english. This is difficult and new to me so i asked my professor, if there is any help to improve creativeness and variety in formulating mathematical sentences in english. His answer was that he guesses by now there should be such literature for mathematical writing in english but he doesn't know any title. I couldn't find any, either.
So here is my question:
Do you know any literature or source for this? (This is not a question about formatting or write down mathematic proofs and results clearly, I am very used to Latex and never had a problem to be unprecise)
If you think this question does not belong here and you know a better place to ask, please tell me.
 A: This doesn't answer your question (I know no literature source), but it does involve some experience (i.e., a career spent reviewing a fair number of papers written by speakers whose first language is not English.)
I strongly recommend that you write your paper in German, and then translate it, a paragraph at a time, to English. 
The organization of thoughts in English, French, German, etc., is pretty similar. I know, because I had to learn to read math papers in French and German back when I did my Ph.D.; after I learned some vocabulary, it was fairly easy. 
The reason to write in German first is that you can concentrate on clearly presenting and explaining the mathematics. That's hard to do in any language, so sticking with your first language helps. It's easy to miss (in a comparatively unfamiliar language) that you left out some step. 
Step 2 is translation. A really useful thing to do here is to pick up a well-written math book or paper in English. (My general advice: anything written by John Milnor.) Look at how introductory paragraphs are written. When you see 

Let $f$ be a function...

appearing again and again, you know (as you translate from French, for instance) that "Soit $f$ une fonction..." must correspond to that. (Google translate, on short fragments, helps a lot here!) You realize that where, in French, you might use the subjunctive, you need to do something else in English, but once you have a pattern/model to work from, it gets a lot easier. 
As a final step, get a native english reader (one who's read some math will be a BIG help) to read through the document and mark up/rewrite awkward sentences. The reader should NOT try to understand the mathematics. The goal should be to make sure it reads like English. Your English appears to be good enough that you can tell whether the rewritten sentence says what you meant.
If you do these three things --- write in German; translate to English with the help of model math-writing; get an English speaker to sanity-check the results --- then when I have to review your paper, I'll be a happy guy. Sure, at some points I'll think to myself "What might this sentence have been in German? Oh...NOW it makes sense!"(**) But mostly, I'll find it not so bad to read. 
(**) I confess, I can't actually do this for German. Sometimes for French, and sometimes it helps. 
Last but not least: include thanks to your English-speaking friend for helping improve the clarity of the document by polishing the English. 
